It is desirable to manufacture instrument dials having rear illumination. Rear illumination reduces glare on the dial surface and also presents a more attractive appearance. Applique overlays have been widely used for rear illuminated dials. Overlays are produced by printing an image on a thin sheet of transparent material. An adhesive is applied to the back side of the overlay and a release paper is placed atop,the adhesive. To manufacture a dial having an overlay, the release paper is removed and the overlay pressed on a clear dial panel substrate.
Overlays are manufactured in individual pieces and then attached to the panel face in a separate assembly process. In an attempt to lower costs and increase the speed of manufacturing instrument dials, dry release ink transfer decals have increasingly been used as a replacement for overlays.
Dry release ink transfer decals are known for making instrument dial face images. An ink transfer layer is applied atop a clear panel substrate usually made of plastic using a heat transfer process. The ink layer is generally carried by a kraft paper. The decal is contacted with the plastic substrate and heat and pressure are applied to adhere the ink transfer to the substrate. The kraft paper carrier is separated from the ink layer so that the ink layer remains adhered to the plastic substrate. The ink layer is extremely fragile. Contact with other objects often mars the dial face image and renders the panel scrap.
The panel undergoes a series of manufacturing operations prior to installation in the vehicle. Electrical and mechanical components are attached to the rear surface of the panel and pointers or needles are attached to it from the dial face surface. Individual dial panels are often assembled into an instrument cluster and installed in the vehicle. All these intermediate assembly steps pose a potential hazard to the fragile instrument dial face. Packaging and handling these fragile dials is slow and cumbersome and increases manufacturing costs.
The largest problem of unprotected ink transfer decals is due to scuffs and scratches. Even slight contact with an object or surface can scuff the ink transfer decal. Scuff marks reflect light differently than the ink layer and are therefore highly visible. Scratches penetrate the ink transfer decal and expose the rear illumination as "light holes."
In an attempt to overcome such difficulties, urethane film layers have been applied atop the ink layer. Such a decal is described in Arnold et al, U.S. Pat. No. 4,308,310. This decal comprises a urethane film and ink layer applied to a backing film. The urethane layer was selected so as to provide flexibility and good adherence to both the ink layer and kraft paper. This type of decal requires the ink transfer layer be applied directly to the urethane film rather than a kraft paper backing. Urethanes do not give the same level of protection as afforded by ultra violet cured hard coats (UV-hard coats). The UV-hard coat provides a very hard surface but does not adhere well to either the kraft paper or ink layer.